There is a basic difference in the way dichroic heads and filter drawers use their filters. A filter in a drawer filters all of the light while a dichro head(subtractive w/moving filters) filters some of the light till you get to the maximum position, leaving some of the light unfiltered most of the time.
A second magenta would in fact reduce the light for the high contrast layer, rebalancing the output toward the low. You would need to use a separate filter to get rid of the light that goes past the edge of the moving filter. Not much- a 40m in the drawer would let you get all the contrast available when added to a fully magenta blocked aperture.